1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting/receiving a signal in an Orthogonal Frequency Division Multiplexing/Orthogonal Frequency Division Multiple Access (OFDM/OFDMA) communication system, and in particular, to an apparatus and method for transmitting/receiving a signal in an OFDM/OFDMA communication system using a plurality of antennas.
2. Description of the Related Art
The basic issue in communications is how efficiently and reliably to transmit data on channels. Along with the demand for a high-speed communication system capable of processing and transmitting video and wireless data in addition to the traditional voice service, increasing system efficiency using an appropriate channel-coding scheme is critical to future-generation multimedia mobile communication systems, which is now under active study.
Generally, in the wireless channel environment of a mobile communication system, unlike a wired channel environment, a transmission signal inevitably experiences loss due to several factors such as multipath interference, shadowing, wave attenuation, time-variant noise, interference, and fading.
The resulting information loss causes a severe distortion to the actual transmission signal, degrading the whole system performance. In order to reduce the information loss, many error control techniques are usually adopted depending on the characteristics of channels to thereby increase system reliability. The basic one of them is to use an error correction code.
Communication instability caused by multipath fading is relieved by diversity schemes. The diversity schemes are classified into time diversity scheme, frequency diversity scheme, and antenna diversity (i.e. spatial diversity) scheme.
The antenna diversity scheme uses multiple antennas. This diversity scheme is further branched into receive (Rx) antenna diversity scheme using a plurality of Rx antennas, transmit (Tx) antenna diversity scheme using a plurality of Tx antennas, Multiple-Input Multiple-Output (MIMO) scheme using a plurality of Tx antennas and a plurality of Rx antennas, and Multiple-Input Single-Output (MISO) scheme. MIMO scheme and MISO scheme are special cases of Space-Time Coding (STC) scheme that extends coding from the time domain to the space domain by transmission of a signal encoded in a predetermined coding scheme through a plurality of Tx antennas, with the aim to achieve lower error rate.
A description will be made of the configuration of a transmitter with four Tx antennas in an OFDM/OFDMA communication system using an STC scheme.
FIG. 1 is a block diagram of a transmitter with four Tx antennas in a typical OFDM/OFDMA communication system using an STC scheme.
Referring to FIG. 1, the transmitter includes an encoder 111, a modulator 113, a space-time mapper 115, first to fourth Radio Frequency (RF) processors 117-1 to 117-4, and first to fourth Tx antennas 119-1 to 119-4.
For the input of information data bits, the encoder 111 encodes them in a predetermined encoding method such as convolutional coding, turbo coding, etc. The modulator 113 modulates the coded bits in a predetermined modulation scheme. The modulation scheme can be one of Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), Pulse Amplitude Modulation (PAM), and Phase Shift Keying (PSK).
The space-time mapper 115 maps the modulation symbols received from the modulator 113 in a predetermined space-time mapping method. Let the modulation symbols be denoted by x1 x2 x3 x4. Then it is assumed that the four modulation symbols x1 x2 x3 x4 form one codeword and four symbol time intervals are taken to transmit one codeword. A time interval for which one codeword is transmitted is called a “codeword transmission time interval”. The space-time mapping is carried out as follows.
                              G          4                =                  [                                                                                          ⅇ                                          jθ                      1                                                        ⁢                                      x                    1                                                                                                x                  2                                                                              x                  3                  *                                                                                                  ⅇ                                          -                                              jθ                        1                                                                              ⁢                                      x                    4                    *                                                                                                                        x                  2                  *                                                                                                  -                                          ⅇ                                              -                                                  jθ                          1                                                                                                      ⁢                                      x                    1                    *                                                                                                                    ⅇ                                          jθ                      1                                                        ⁢                                      x                    4                                                                                                -                                      x                    3                                                                                                                        x                  3                                                                                                  ⅇ                                          jθ                      1                                                        ⁢                                      x                    4                                                                                                                    -                                          ⅇ                                              -                                                  jθ                          1                                                                                                      ⁢                                      x                    1                    *                                                                                                -                                      x                    2                    *                                                                                                                                            ⅇ                                          -                                              jθ                        1                                                                              ⁢                                      x                    4                    *                                                                                                -                                      x                    3                    *                                                                                                -                                      x                    2                                                                                                                    ⅇ                                          jθ                      1                                                        ⁢                                      x                    1                                                                                ]                                    (        1        )            where G4 represents a coding matrix for symbols transmitted through the four Tx antennas. The columns and rows of the coding matrix represent the Tx antennas and symbol time intervals, respectively.
Thus, for a first symbol time interval t1, ejθ1x1 is transmitted through the first Tx antenna 119-1, x2 through the second Tx antenna 119-2, x*3 through the third Tx antenna 119-3, and e−jθ1x*4 through the fourth Tx antenna 119-4. For a second symbol time interval t2, x*2 is transmitted through the first Tx antenna 119-1, −e−jθ1x*1 through the second Tx antenna 119-2, ejθ1x4 through the third Tx antenna 119-3, and −x3 through the fourth Tx antenna 119-4. For a third symbol time interval t3, x3 is transmitted through the first Tx antenna 119-1, ejθ1x4 through the second Tx antenna 119-2, −e−jθ1x*1 through the third Tx antenna 119-3, and −x*2 through the fourth Tx antenna 119-4. For a fourth symbol time interval t4, e−jθ1x*4 is transmitted through the first Tx antenna 119-1, −x*3 through the second Tx antenna 119-2, −x2 through the third Tx antenna 119-3, and ejθ1x1 through the fourth Tx antenna 119-4.
The space-time mapper 115 maps the modulation symbols using the coding matrix G4, so that they can be transmitted through the corresponding Tx antennas for the corresponding symbol time intervals.
Specifically, for the first symbol time interval t1, the space-time mapper 115 outputs ejθ1x1 to the first RF processor 117-1, x2 to the second RF processor 117-2, x*3 to the third RF processor 117-3, and ejθ1x*4 to the fourth RF processor 117-4. For the second symbol time interval t2, the space-time mapper 115 outputs x*2 to the first RF processor 117-1, −e−jθ1x*1 to the second RF processor 117-2, ejθ1x4 to the third RF processor 117-3, and −x3 to the fourth RF processor 117-4. For the third symbol time interval t3, the space-time mapper 115 outputs x3 to the first RF processor 117-1, ejθ1x4 to the second RF processor 117-2, −e−jθ1x*1 to the third RF processor 117-3, and −x*2 to the fourth RF processor 117-4. For the fourth symbol time interval t4, the space-time mapper 115 outputs e−jθ1x*4 to the first RF processor 117-1, −x*3 to the second RF processor 117-2, −x2 to the third RF processor 117-3, and ejθ1x1 to the fourth RF processor 117-4.
The first to fourth RF processors 117-1 to 117-4 process the received signals to RF signals and transmit them through the corresponding Tx antennas. That is, the first RF processor 117-1 is mapped to the first Tx antenna 119-1, the second RF processor 117-2 to the second Tx antenna 119-2, the third RF processor 117-3 to the third Tx antenna 119-3, and the fourth RF processor 117-4 to the fourth Tx antenna 119-4.
Besides the information data, a reference signal for channel estimation is transmitted through the first to fourth Tx antennas 119-1 to 119-4 for the codeword transmission time interval, while not shown in FIG. 1. The reference signal is a pilot signal, for example. A pilot signal transmitted through each of the Tx antennas 119-1 to 119-4 occupies a different frequency domain, i.e. different subcarriers, and each of the Tx antennas 119-1 to 119-4 has to discontinue pilot transmission in subcarrier areas for the other Tx antennas. The subcarriers carrying the pilot signal are called “pilot subcarriers”.
When a plurality of Tx antennas are used as illustrated in FIG. 1, a Tx antenna diversity gain corresponding to the number of the Tx antennas is achieved. However, the use of frequency diversity scheme limits the Tx antenna diversity gain.
In the OFDM/OFDMA communication system using a plurality of Tx antennas, limited resources, i.e. pilot subcarriers are divided for the Tx antennas, thereby degrading channel estimation performance. In addition, as many RF processors as the number of the Tx antennas are required, thus increasing hardware complexity and cost.
Accordingly, there exists a need for a signal transmitting/receiving method for minimizing hardware complexity and cost, while improving channel estimation performance in an OFDM/OFDMA communication system with a plurality of Tx antennas.